Hi OP. My story is similar to yours except that I have been TTC my first. Unexplained sub fertility, tried for just over 2 years then had my first round of IVF not long before my 38th birthday. (It was unsuccessful but I am currently 11 weeks pregnant with a frozen embryo from the first cycle.) I also temped and had acupuncture for most of our second year of trying. And would confess to being a teeny tiny bit of a control freak and a wworrier.
I reckon slapping anyone who tells you "relax and it'll happen" or variations thereof would be a great way to relieve stress :-)
However, I think there is a kernel of sense in it, in that like lozster says, you get pregnant (if you get pregnant) because physically there was always a chance you would. It probably doesn't make much difference to the outcome whether or not you spend the intervening months stressed out of your head, unless it is "life in a war zone" level of stress. But it will make a difference to how your life and relationships are, so it is better for your overall health to let yourself off the hook a bit if you can.
I found it helped to shift my focus slightly. I researched when the probability of conceiving naturally would drop below our probability of a successful outcome through IVF, which in our case was after 2 years of trying. DH and I decided in our own minds, as it seems you have done, to move on to IVF at that point. Knowing our plan B was decided upon helped me be calmer about leaving it to fate in the meantime. Also, rather than focus on "trying" during that period I attempted (not always successfully) to focus on getting healthy, doing yoga for relaxation, spending time with DH doing things we enjoy, etc, so that we would be in the best place we could, physically and mentally, for IVF if we had to use plan B.
I think that it is natural that if you tell yourself not to think about a particular thing, it will be all you think about, so "trying not to try", as it were, will be doomed to failure! But distracting the mind with other things might help.
I had the same experience as lozster with the AMH test, by the way - the NHS hospital gave me the option of sending me to a private clinic for the AMH test, for which I paid £40.